The field of the invention is programmable controllers such as that disclosed in U.S. Pat. No. 3,810,118 issued to William W. Kiffmeyer on May 7, 1974 and entitled "Programmable Matrix Controller".
Programmable controllers are typically connected to industrial equipment such as assembly lines and machine tools to sequentially operate the system in accordance with a stored control program. In programmable controllers such as that disclosed in the above cited patent, for example, the control program is stored in a random access memory and includes sets of instructions which are executed in rapid sequence to examine the condition of selected input devices on the controlled system and instructions which energize or deenergize selected output devices on the controlled system contingent upon the status of one or more of the examined input devices. The response of the controlled system to changing conditions is directly related to the time it takes to scan the entire control program, and because the control program is executed at a fixed rate, the time necessary to cycle through it is a constraint on the complexity of the system which can be controlled by the programmable controller.
Although the time necessary to scan through a given control program can be decreased by increasing the rate at which the program instructions are executed, there is a limit to such an approach in programmable controllers such as that disclosed in the above cited patent. More specifically, the rate at which input information is received by the programmable cntroller processor from its input circuits and the rate at which it generates output information to its output circuits is limited by propagation times and the time delays which are associated with the noise immunity circuitry. Therefore, in prior programmable controllers a compromise is made between these conflicting objectives with the result that the program instruction execution rate is reduced and the noise immunizing circuits are kept to a minimum.